Experimental and Modeling Evaluation of Dimethoxymethane as an Additive for High-Pressure Acetylene Oxidation

The high-pressure oxidation of acetylene–dimethoxymethane (C2H2–DMM) mixtures in a tubular flow reactor has been analyzed from both experimental and modeling perspectives. In addition to pressure (20, 40, and 60 bar), the influence of the oxygen availability (by modifying the air excess ratio, λ) and the presence of DMM (two different concentrations have been tested, 70 and 280 ppm, for a given concentration of C2H2 of 700 ppm) have also been analyzed. The chemical kinetic mechanism, progressively built by our research group in the last years, has been updated with recent theoretical calculations for DMM and validated against the present results and literature data. Results indicate that, under fuel-lean conditions, adding DMM enhances C2H2 reactivity by increased radical production through DMM chain branching pathways, more evident for the higher concentration of DMM. H-abstraction reactions with OH radicals as the main abstracting species to form dimethoxymethyl (CH3OCHOCH3) and methoxymethoxymethyl (CH3OCH2OCH2) radicals are the main DMM consumption routes, with the first one being slightly favored. There is a competition between β-scission and O2-addition reactions in the consumption of both radicals that depends on the oxygen availability. As the O2 concentration in the reactant mixture is increased, the O2-addition reactions become more relevant. The effect of the addition of several oxygenates, such as ethanol, dimethyl ether (DME), or DMM, on C2H2 high-pressure oxidation has been compared. Results indicate that ethanol has almost no effect, whereas the addition of an ether, DME or DMM, shifts the conversion of C2H2 to lower temperatures.


Model performance for experimental data sets found in literature a. Dimethoxymethane oxidation in a jet-stirred reactor (JSR)
Experiments reported by Vermeire et al. 23 performed in a quartz jet-stirred reactor have also been used to validate the kinetic mechanism. Four different equivalence ratios have been investigated, ø=∞ (pyrolysis), ø=2, ø=1 and ø=0.25. Simulations have been performed with the continuous stirred-tank reactor of the Chemkin-Pro software package 38

b. Dimethoxymethane oxidation in an atmospheric-pressure tubular-flow reactor
The kinetic mechanism has been validated with experiments reported by Marrodán et al. 21 performed in a tubular-flow reactor at atmospheric pressure from pyrolysis to fuel-lean conditions, i.e. the air excess ratio was varied from λ=0 to λ=35. Simulations have been performed with the plug-flow reactor module of the Chemkin-Pro software package 38

c. Dimethoxymethane oxidation in a high-pressure tubular-flow reactor
The kinetic mechanism has also been validated with experiments reported by Marrodán et al. 20 performed in a tubular-flow reactor at high pressure (20-60 bar). The air excess ratio was varied from λ=0.7 to λ=20. Simulations have been performed with the plug-flow reactor module of the Chemkin-Pro software package 38 20 , and modeling calculations (lines) obtained with the present mechanism are compared for different air excess ratios (λ=0.7, λ=1 and λ=20).   20 , and modeling calculations (lines) obtained with the present mechanism are compared for different air excess ratios (λ=0.7, λ=1 and λ=20).

d. Ignition delay times of DMM
Ignition delay times reported by Li et al. 26 measured in a shock tube at pressures 1 and 4 atm, for equivalence ratios of 0.5, 1 and 2 have also been used to validate the kinetic mechanism.
Simulations have been performed with the closed homogeneous reactor of the Chemkin-Pro software package 38 . Results of the comparison of model calculations and experimental data are shown in Figures S19 and S20. Experimental results (symbols) reported by Li et al. 26 , and modeling calculations (lines) obtained with the present mechanism are compared for different equivalence ratios (ø=0.5, ø=1 and ø=2) and 1 atm. Experimental results (symbols) reported by Li et al. 26 , and modeling calculations (lines) obtained with the present mechanism are compared for different equivalence ratios (ø=0.5, ø=1 and ø=2) and 4 atm.

e. Acetylene oxidation in a high-pressure tubular-flow reactor
Experiments reported by Giménez et al. 39 , performed in a tubular-flow reactor, have also been used to validate the kinetic mechanism. Two different air excess ratios (λ=0.99 and λ=19.4) for two pressures, 59.6 and 49.6 bar, respectively, have been tested. Simulations have been performed with the closed homogeneous reactor of the Chemkin-Pro software package 38 by fixing the gas residence time inside the reactor. Results of the comparison of model calculations and experimental data are shown in Figure S21.  Figure S21. Acetylene concentration as a function of temperature during its oxidation at highpressure (59.6 and 49.6 bar). Experimental results (symbols) reported by Giménez et al. 39 , and modeling calculations (lines) obtained with the present mechanism are compared for different air excess ratios (λ=0.99 and λ=19.4).